In the Kraft process, cellulosic fibrous feed material (wood) is heated in a digestion stage with a "white liquor" which contains sodium sulfide and sodium hydroxide to dissolve hemicelluloses, lignin and other extractable or organic materials contained in the fibrous material. The digested fibrous pulp so obtained is separated from the resultant "black liquor", the latter being sent to a recovery stage and the former being sent to a bleaching stage. The above process is of particular application to the pulping of wood chips.
In the recovery stage, the black liquor is concentrated by evaporation of water therefrom and the concentrated liquor is then burned in a boiler (sometimes referred to as a "furnace") to yield a smelt containing sodium carbonate and sodium sulfide. The smelt is quenched with water to form a raw "green liquor" which is then clarified. The clarified green liquor is causticized with lime to convert the sodium carbonate present in the liquor to sodium hydroxide. Calcium carbonate is precipitated during the causticization of the liquor and is separated therefrom as a mud and calcined to regenerate lime for further causticization. The filtered causticized green liquor is the white liquor which is used in the digestion stage and is recycled to treat further fibrous material.
An appreciable portion of the inorganic chemicals contained in the black liquor are lost to the recovery boiler flue gas, either by entrainment or volatilization. To conserve chemicals it is common practice to collect dust by electrostatic precipitation (ESP dust) from the boiler exhaust, and recycle the dust by redissolution of same in the black liquor. Additional sodium sulfate is usually added to the process to make up any net loss of chemicals in the cycle.
In the Kraft process described above, sodium chloride and other impurities entering with the wood and input chemicals tend to build up to a steady state concentration in the pulping liquors since they have no deliberate outlet. This is a particular problem for coastal mills such as those located on the coast of British Columbia where logs are transported in seawater and become saturated in chloride. Other than in exceptional cases such as these coastal mills, chloride contamination has not been a serious problem in the past, as impurity outlets occur due to normal spills, leakages and other losses. As mills improve their operating practices however, there is a reduced outlet for contamination and an increasing tendency for build-up of impurities. Means of removal of contaminants is becoming increasingly necessary.
The pulp produced in the Kraft mill is subjected to bleaching and purification in a bleach plant operation. Treatment chemicals, commonly in aqueous solutions, and wash water are used in the bleaching and purification of the pulp, and result in one or more aqueous bleach plant effluents containing spent chemicals and spent wash water. Such bleach plant effluents usually are discharged, after treatment, to water bodies. The treatment of bleach plant effluents represents considerable expense. The possibility of recycling bleach plant effluents to eliminate their discharge and associated treatment is very attractive and has been an industry goal for some time. Recycling of bleach plant effluent would go a long way towards elimination of pollution from pulp mill operations.
In 1972, Rapson patented (U.S. Pat. No. 3,698,995) a process for reducing the discharge of the bleach plant effluent by recycling the filtrates to the pulping liquor regeneration process. This process was later improved by Reeve et al. in U.S. Pat. No. 4,039,372. The Rapson/Reeve process was installed at Great Lakes Paper in Thunder Bay, Ontario and began operation in 1977 but was abandoned in 1985. One of the main reasons for the apparent failure of this process was related to concerns regarding corrosion and pulp quality because of inadequate removal of chloride contamination. Chloride originating from chlorine based chemicals used in pulp bleaching, was recycled with the bleach filtrate to the kraft recovery process from whence it had no adequate outlet.
The Rapson/Reeve process included some provision for chloride removal by evaporation and crystallization of sodium chloride from white liquor. This chloride removal process did not prove sufficient, however.
In the Kraft pulping chemical recovery cycle, it is well-known that chloride and potassium become enriched relative to sodium sulfate in the flue gas dust retained by the electrostatic precipitator in the recovery boiler. These elements decrease the melting point of the dust, leading to plugging of the boiler tubes, which leads to decreased boiler efficiency. This impurity enrichment has an beneficial aspect, however and several prior art processes have taken advantage of this enrichment to facilitate removal of chloride and potassium impurities from the Kraft pulping process.
A improved process for recycling bleach plant filtrate (called BFR) is disclosed in U.S. Pat. No. 5,352,332 (Maples et al.) and was recently installed at the Canton N.C. mill of Champion International. A key feature of the BFR process is its ability to remove chlorine and potassium contamination from the Kraft pulping process by treatment of the electrostatic precipitator (ESP) dust catch. According to the patent this is done by selectively leaching the chloride and potassium from the dust with a minimal amount of water or by recrystallizing sodium sulfate after dissolution of the dust in water.
The leaching technique is described in more detail by Moy et al. ("Removal of Sodium Chloride from Kraft Recovery System": Pulp Paper Mag. Can. 75(4): T150 (April 1974)). The basic principle of this leaching process is that sodium chloride is much more soluble that sodium sulfate. In fact, the solubility of sodium sulfate in a saturated solution of sodium chloride is significantly reduced, according to the well-known common ion effect. The idea is to add just enough water to dissolve the sodium chloride, leaving most of the sodium sulfate behind. In practice this idea is unworkable as it produces a thick non-pumpable paste containing 45-60% undissolved solids. One reason that this material is difficult to filter is the presence of fine particles of oxides of non-process elements such as calcium, manganese, iron, zinc etc which are mixed in with the sodium salts.
Moy found that by decreasing the solids concentration to 20-25% the slurry became workable. However the dilution water dissolved much more sodium sulfate, which would then be lost to waste with the sodium chloride. This problem was partially overcome by using a saturated solution of sodium chloride/sodium sulfate instead of water to dilute the thick slurry. In practice, the slurry is dewatered using, for example, a rotary drum vacuum filter. A portion of the filtrate, which is a saturated solution of sodium chloride/sodium sulfate is then recycled for use in diluting the slurry.
The other problem with the basic leaching process is that sodium carbonate, a valuable component of the dust, is leached out of the liquor along with the sodium chloride. Moy overcame this problem by converting the sodium carbonate to sodium sulfate by neutralizing the salt slurry to a pH of 3.5 to 5.5 using sulfuric acid. This additional sodium sulfate was largely retained with the solids removed by the filter.
When Champion International installed the BFR system discussed above, they determined that the dust leaching process described above was not satisfactory. Several shortcomings were cited. According to Earl ("Removal of Chloride and Potassium from the Kraft Recovery Cycle": Paper by Paul F. Earl, P. David Kick and Jean-Claude Patel), the leaching process does not effectively remove potassium from the system. In addition, the amount of sodium sulfate lost with the sodium chloride in the filtrate was still considered excessive.
A new process, developed by Sterling Pulp Chemicals, was installed at Champion wherein all of the dust is dissolved in water. Sulfuric acid is added to convert sodium carbonate to sodium sulfate. The liquor is then concentrated by evaporation. Sodium sulfate is then recrystallized from the concentrated liquor and separated from the sodium chloride liquor by filtration. A portion of the filtrate, containing sodium chloride as well as potassium chloride is purged from the system. According to Earl, the amount of sodium sulfate lost is much less with this system than with the leaching process of the prior art. It is claimed that this process recovers 90% of the sodium sulfate values in the dust while removing 90% of the chloride and greater than 60% of the potassium. In addition, it is claimed that the relatively coarse sodium sulfate crystals are much easier to filter than the fine dust particles of the leaching process.
While apparently quite effective in its operation, the major disadvantage of the recrystallization process is the high capital and operating cost associated with the evaporator employed to concentrate the dissolved dust liquor.
A common problem with all of the above described prior art processes is that non-process elements such as calcium , manganese, iron and zinc which are present in the ESP dust are largely insoluble in water. As a result they are filtered out along with the recovered sodium sulfate and recycled back to the kraft pulping process. It is well known that these non-process elements are undesirable and a process which also removes these non-process elements would be preferred.